A lithium ion battery and a fuel cell that are used as a power source for portable devices have been actively studied in recent years. A solid electrolyte used for the above mentioned battery or cell is also actively studied. The solid electrolyte is, for instance, a lithium ion conducting material or a proton conducting material.
The proton conducting material is generally in the form of a membrane. The solid electrolyte in membrane form, which is used as a solid electrolyte layer of the fuel cell and the like, and its producing method have been proposed. For instance, Japanese Patent Laid-Open Publication No. 9-320617 discloses a method of producing a solid electrolyte membrane by immersing a polyvinylidene fluoride resin in a liquid in which an electrolyte and a plasticizer are mixed. Japanese Patent Laid-Open Publication No. 2001-307752 discloses a method of producing a proton conducting membrane by synthesizing an inorganic compound in a solution containing an aromatic polymer compound with the sulfonic acid group, and removing a solvent therefrom. In this method, oxides of silicon and phosphoric acid derivative are added to the solution in order to improve micropores. Japanese Patent Laid-Open Publication No. 2002-231270 discloses a method of producing an ion-exchange membrane. In this method, metal oxide precursor is added to a solution containing an ion-exchange resin, and a liquid is obtained by applying hydrolysis and polycondensation reaction to the metal oxide precursor. The ion-exchange membrane is obtained by casting the liquid. Japanese Patent Laid-Open Publication No. 2004-079378 discloses a method of producing a proton conducting membrane. In this method, a polymer membrane with a proton conductivity is produced by a solution casting method. The membrane is immersed in an aqueous solution of an organic compound soluble to water and having a boiling point of not less than 100° C., and is allowed to swell to equilibrium. Water is then evaporated by heating. In this way, the proton conducting membrane is produced. Japanese Patent Laid-Open Publication No. 2004-131530 discloses a method of producing a solid electrolyte membrane by dissolving a compound consisting essentially of polybenzimidazole having the anionic groups into an alcohol solvent containing tetraalkylammonium hydroxide and having a boiling point of not less than 90° C. Japanese Patent Laid-Open Publication No. 2005-146018 proposes a method of producing a proton conducting membrane. In this method, a coating liquid including (a) a polymer containing ion conducting component, (b) water-soluble inorganic compound or water-soluble organic compound whose molecular weight is less than 1000 and (c) organic solvent is applied to a support. Then, a dry coating layer is formed by removing (c) organic solvent. Thereafter, the proton conducting membrane is produced by removing (b) water-soluble compound.
A melt-extrusion method and the solution casting method are well known methods of forming a membrane from a polymer.
According to the melt-extrusion method, the membrane can be formed without using a solvent. However, this method has problems in that the polymer may denature by heating, impurities in the polymer remain in the produced membrane, and the like. On the other hand, the solution casting method has a problem in that its producing apparatuses become large and complicated since the method requires a producing apparatus of a solution, a solvent recovery device and the like. However, this method is advantageous since a heating temperature of the membrane can be relatively low and it is possible to remove the impurities in the polymer while producing the solution. The solution casting method has a further advantage in that the produced membrane has better planarity and smoothness than the membrane produced by the melt-extrusion method.
Japanese Patent Laid-Open Publication No. 2005-146018 discloses a method of producing a solid electrolyte membrane by the solution casting method. In this method, a solution containing a polymer having an acid group and a solvent is cast on a support to form a casting membrane. The casting membrane is dried at temperatures of a predetermined value or lower and peeled from the support. The peeled membrane is dried again by heating. In this way, the solid electrolyte membrane is produced.
The solvent of the solid electrolyte has high polarity. The solvent is often basic substance that easily acts with protons. When a solid electrolyte membrane in which the basic substances remained is used for a fuel cell, the basic substances may prevent the protons from passing through the solid electrolyte membrane. As a result, an ionic conductivity of the solid electrolyte membrane is lowered, and the fuel cell cannot express enough electromotive force. In the solution casting method, each drying step is performed over a casting membrane or a membrane in order to remove the solvent from the casting membrane or the membrane. However, the solvent of the solid electrolyte generally has a high boiling point. Therefore, it is often difficult to form the membrane from which the solvent is completely removed by the drying step.
In the above-noted Publication No. 9-320617, the solution casting method is denied and there remains a problem in that the impurities contained in raw materials remains in the produced membrane. The methods disclosed in the above-noted Publication Nos. 2001-307752, 2002-231270, 2004-079378 and 2004-131530 are on a limited scale and not intended to be applied in mass production. The method disclosed in the above-noted Publication No. 2001-307752 has a problem in that it is difficult to disperse a complex consisted of the polymer and the inorganic compound. The method disclosed in the above-noted Publication No. 2002-231270 has a problem in that its membrane producing step is complicated. The method disclosed in the above-noted Publication No. 2004-079378 has a problem in that the produced membrane is not uniform in planarity and smoothness since it has micropores formed during the immersing in the aqueous solution. Any solution for this problem is not cited in the disclosure. Although it is cited in the disclosure that various solid electrolyte membranes can be produced by the solution casting method, any specific method therefor is not cited. The method disclosed in the above-noted Publication No. 2004-131530 limits raw materials to be used and does not mention the usage of other materials having excellent properties.
According to the method disclosed in the above-noted Publication No. 2005-146018, it takes time to dry the casting membrane. In order to produce the membrane continuously, it is necessary to either (1) use a support having long length or (2) regulate running speed of the support slow. Option (1) makes the apparatus large in size, and option (2) lacks production efficiency. Therefore, this method is not preferable for the continuous membrane production. In the method of this publication, the coating layer containing (a), (b) and (c) was dried to remove (b) water-soluble compound. However, (b) generally has a high boiling point. Therefore, it is difficult to remove (b) by drying. The disclosure does not mention any solution for facilitating the removal of the organic solvent with high boiling point by drying.
It is an object of the present invention to provide a solid electrolyte membrane that has uniform quality and excellent proton conductivity continuously formed from a solid electrolyte, a method and an apparatus of producing the solid electrolyte membrane, and a membrane electrode assembly and a fuel cell using the solid electrolyte membrane.